Experimental aircraft is a term used to refer to aircraft which have not been proven fully in flight. However, experimental aircraft has become a common reference for homebuilt aircraft. Experimental homebuilt aircraft (“homebuilt aircraft”) are constructed by a homebuilder; that is, homebuilt aircraft are not built by a licensed aircraft manufacturer. Generally, about 51% of a homebuilt aircraft is constructed by a private individual; the remaining portion of homebuilt aircraft is usually from a kit that is assembled by a manufacturer. The fuel injection system of a homebuilt aircraft is bought from the manufacturer.
Homebuilt, or any other aircraft, have either a Multi-Point Injection System (“MPIS”) or a carburetor. Smaller aircraft commonly have a carburetor. In a MPIS one injector supplies fuel directly to a cylinder of the engine. In a Single-Point Fuel Injection System (“SPIS”), fuel is injected at a single place and then distributed to each cylinder of the engine.
Fuel injection systems are designed to meter fuel in direct ratio to the volume of air being consumed by the engine at any given time. Generally, an engine driven pump receives fuel from the fuel tank and supplies that fuel to a fuel injection servo. Fuel injection servos are well known in the art. The “RSA Fuel Injection System, Training Manual” written by Precision Airmotive Corporation, is hereby incorporated, in its entirety, by reference.
Fuel injection servos are tuned in the factory before shipment to the homebuilder. However, because homebuilt aircraft come in varying sizes, the fuel injection servo may need to be fine tuned for optimal results. A fuel injection servo will get peak performance when a maximum air pressure differential signal is received by the inlet of the servo. Prior to leaving the factory, a fuel injection servo is tuned to a standard differential air pressure. Because of tolerances allowed in manufacture of the servos, the shape of the venturi (500) will have minor variance.
FIGS. 1 and 1A show a fuel injection servo known in the art. The size of the venturi is definite. To obtain a standard differential air pressure, the venturi will sometimes be filed down by hand. Once the shape of the venture is changed, its performance can only be verified with the proper airflow equipment. This cannot be done in the field.
Referring to FIG. 2, the idle valve is connected to the throttle linkage. The idle valve effectively reduces the area of the main metering jet for accurate metering of the fuel in the idle range. The idle control valve is opened/closed by rotating a flat metal plate over the valve's opening. As with any mechanical function that creates a metal on metal situation, the idle control valve starts to wear.
Referring to FIG. 3, to ensure that the idle valve is properly seals, a strong spring is used to hold the valve in place. To fine tune the idle valve, the spring must be changed. When the idle valve bears a higher load, caused by the spring, the idle valve tends to wear quicker.
Fuel injection servos for homebuilt aircraft are normally MPIS. Smaller aircraft generally have carburetors. The carburetor has several deficiencies. First, carburetor icing becomes a problem. Carburetor icing is caused by a change in temperature due to fuel vaporization prior to entering the carburetor. Vaporizing fuel can also cause the throttle valve of the carburetor to freeze. This scenario leaves the engine without air. The homebuilder can manage this weakness in the carburetor by installing a heating device for the carburetor. However, small aircraft may not have room for a heating device. Further, heating devices cause power loss and need constant pilot attention. Second, carburetors are sensitive normal operations. Third, it is difficult to adjust a carburetor to optimize fuel flow. Finally, an aircraft cannot fly upside down with a carburetor because airflow through the carburetor can go only one direction. Replacing a carburetor with a fuel injection system would solve these problems. However, a MPIS does not exit for smaller planes. Conceivably, the MPIS could be adapted, after market, for the smaller aircraft. However, a better solution is a SPIS which is made for the smaller aircraft.
Another problem that smaller aircraft face is delayed response at sudden throttle opening or acceleration. This is a natural occurrence in smaller aircraft because the fuel discharge point is further away from the cylinders.
The invention, described herein, is an improved Fuel Injection Servo (“Servo”) for the homebuilt aircraft. The Servo has been designed to allow the manufacturer to more easily fine tune the pressure differential over the air diaphragm. The Servo also provides an idle valve that the manufacturer and homebuilder can easily fine tune. In a second embodiment, the Servo is further adapted to replace the carburetor in smaller aircraft.